1. Field of Technology
The present invention relates to an improvement in a semiconductor laser and method of making the same. Especially the present invention relates to a semiconductor laser made by epitaxial growth method.
2. Prior Art
Hitherto various proposals on configuration and methods of making have been made concerning semiconductor lasers for stable fundamental transverse mode operation. In general, the fundamental transverse operation can be obtained by making the width of the stripe shaped active region narrow thereby confining only a lowest mode light in the narrow stripe shaped active region.
A most typical example of conventional fundamental transverse mode operation has a buried stripe layered structure as shown in FIG. 1. Making of the conventional laser of FIG. 1 is as follows:
Firstly, the following layers are subsequently formed by epitaxial growth method
on a substrate 1 of n.sup.+ -GaAs: PA0 on a substrate 8 of n.sup.+ -GaAs: PA0 and an electrode containing layer 12 of p.sup.+ -GaAs. PA0 on a substrate 15 of n.sup.+ -GaAs: PA0 and an active layer 17 of non-doped GaAs.
a first clad layer 2 of n-Ga.sub.1-x Al.sub.x As, PA1 an active layer 3 of n-GaAs, PA1 a second clad layer 4 of p-Ga.sub.1-x Al.sub.x As and PA1 an electrode contacting layer 5 of p.sup.+ -GaAs. PA1 a specially shaped first clad layer 9 of n-Ga.sub.1-x Al.sub.x As, PA1 an active layer 10 of n-GaAs, PA1 a second clad layer 11 of p-Ga.sub.1-x Al.sub.x As, PA1 a first clad layer 16 of n-Ga.sub.1-x Al.sub.x As, PA1 a second clad layer 18 of p-Ga.sub.1-x Al.sub.x As PA1 and an electrode contacting layer 19 of p.sup.+ -GaAs.
Secondly, an SiO.sub.2 film (not shown) is formed on all of the face of the electrode contacting layer 5, then the SiO.sub.2 film is etched by known photo-etching method retaining a stripe shape part, and then by utilizing the stripe shape SiO.sub.2 film as an etching mask, the abovementioned epitaxial growth layers 2 to 5 are mesa-etched to form a stripe shaped part.
Thirdly, into the mesa-etched spaces on both sides of the stripe shaped part are formed filled-in layers 6,6' having a high specific resistance and wider energy gap than those of layers 2 to 5 of the stripe shaped part.
Then on the electrode contacting layer 5 and the filled-in layers 6,6' is formed an ohmic electrode 7, and another ohmic electrode 7 is formed on the bottom face of the substrate 1.
The abovementioned conventional laser of FIG. 1 has the following shortcomings.
A first shortcoming is that there is a possibility of contamination and/or oxidation of side faces of the stripe part during the time between the step of mesa-etching to form the stripe part and the subsequent step of forming filled-in layers 6,6'. Especially, since the side faces of the active layer 3 serve as side faces of a cavity of laser, such contamination and/or oxidation of the side faces of the striped part is likely to form dark spots of the laser and causes a deterioration of the laser.
A second shortcoming is that, since the SiO.sub.2 film is retained on the striped part during a high temperature processing step for the forming of the filled-in layers 6', a difference of thermal coefficient between the SiO.sub.2 film and the p.sup.+ -GaAs layer 5 produces a stress of the active layer 3, thereby forming a cause of deterioration.
A third shortcoming is that such an impurity as Fe or Cr contained in the filled-in layers 6,6' for giving the layer a deep energy level is likely to diffuse into the side faces of the active layer 3, thereby deteriorating the characteristics.
A fourth shortcoming is poor heat radiation due to filling of high resistivity layer 6,6' which has poor heat conductivity.
Recently, some improvements have been proposed in order to eliminate the abovementioned shortcomings of the conventional semiconductor laser of buried-in stripe shaped active region. One example of such improved structure is shown in FIG. 2. The conventional device of FIG. 2 is made as follows:
Firstly, the following layers are sequentially formed by epitaxial growth method
The shape of the first clad layer 9 is, as shown in FIG. 2, has a stripe shaped thicker part 9' at the central part and a thinner part 9", 9" on both sides of the thicker part 9'.
Secondly, an SiO.sub.2 film 13 is formed on all the face of the electrode contacting layer 12, and then the SiO.sub.2 film 13 is etched by a known photo-etching method to form a stripe shaped opening 13' over the position on the stripe shaped thicker part 9', thereby exposing a stripe shaped part of the surface of the electrode contacting layer 12.
Thirdly, an ohmic electrode 14 is formed on the face of the SiO.sub.2 film 13 and on the exposed surface of the electrode contacting layer 12.
The conventional laser of FIG. 2 can operate a fundamental mode lasing in the part of the active layer 10 which is on the thicker part 9', since lights leaking from the side parts of the active layer 10 passes through the thinner parts 9",9" and are absorbed in the substrate 8.
However, the abovementioned conventional laser of FIG. 2 has the shortcoming that the active region is flat, and hence, the light lased at the central part of the active layer 10 is likely to diverge in the horizontal direction of FIG. 2. Therefore, the lasing mode is distorted widthwise. Furthermore, the device of FIG. 2 has a poor heat radiation since the SiO.sub.2 film 13 extends over most of the area between the electrode contacting layer 12 and the ohmic electrode 14.
Another prior art of FIG. 3 has been known. The device of FIG. 3 has a rib shape active layer 17 having a stripe pattern thicker part 171 at the central part and thinner parts 17', 17' on both sides thereof. The conventional semiconductor laser with rib shape active layer 17 is made as follows:
Firstly, the following layers are subsequently formed by epitaxial growth method
Secondly, the thinner parts 17', 17' are formed by slightly etching the active layer 17 by utilizing a striped shape mask of known material, thereby forming the rib shape active layer 17.
Thirdly, again by the epitaxial growth method, the following layers are subsequently formed on the rib shape active layer 17:
Fourthly, an SiO.sub.2 film 20 is formed on all the face of the electrode contacting layer 19, and then, the SiO.sub.2 film 20 is etched by a known photo-etching method to form a stripe shaped opening 201 over the position on the stripe shape thicker part 171, thereby exposing a stripe shaped part of the surface of the electrode contacting layer 19.
Fifthly, an ohmic electrode 21 is formed on the face of the SiO.sub.2 film 10 and on the exposed surface of the electrode contacting layer 19.
The conventional laser of FIG. 3 has the advantage that by means of partly thickened active layer 17, the thicker part 171 and the thinner parts 17', 17' has different values of effective refractive index. Therefore, the lased light can be effectively confined in the thicker part 171, and therefore, a stable transverse mode lasing is possible.
However, the conventional laser of FIG. 3 has the shortcomings that, since the etching of the active layer 17 is made to form the thinner parts 17', 17', two sequences of epitaxial growths and delicate controlling of the etching are necessary, and further that there is a considerable possibility of introducing a number of non radiative centers to the interface between the surface of the active layer 17 and the second clad layer 18 during exposing and etching of the active layer 17.